The purpose of this proposal is to determine the degree of association between Infantile Autism and the Fragile X(fra X) Syndrome. Our group has identified several males with the fra(x) Syndrome who had been previously diagnosed as having Infantile Autism. To further establish the nature of this relationship, 200 individuals who meet the DSM III (1980) critera for Infantile Autism will be screened for the presence of fra(X) Syndrome by the Department of Human Genetics. All siblings of the affected individuals will also be examined. The physical, neurological, psychophysiological, pathogenetic and behavioral characteristics of the affected autistic individuals will be compared with a matched group of autistic individuals who do not exhibit autistic behavior. To date, we have identified 35 hemizygotes and 14 heterozygous carriers from 21 families. The average fra(X) frequency in affected males was 32 percent with a range of 4-70%. Thus far, over 200 individuals have been screened for the fra(X) chromosome, six of 16 autistic individuals have been identified as having the fra(X) chromosome.